Gallium nitride (GaN) is a wide-gap semiconductor and is applied to an LED, a laser, a power semiconductor, an optical device, and others. A high-quality GaN crystal is required in order to manufacture a high-performance semiconductor device by using GaN.
Several methods have heretofore been used for growing a GaN crystal. For example, a molecular beam epitaxy (MBE) method and a metal organic chemical vapor deposition (MOCVD) method are used for growing a crystal up to several microns in thickness. Further, a hydride vapor phase epitaxial growth (HVPE) method and the like are used for growing a bulk GaN crystal exceeding ten microns in thickness.
In a bulk GaN crystal manufactured by an HVPE method or an epitaxially grown GaN crystal manufactured by an MBE method or an MOCVD method, however, the existence of a nanovoid is recognized (Non-patent Literatures 1 to 3). It has been reported that usually nanovoids of about 105 to 107 cm−2 exist in a crystal, and a relevance with O impurity or an Si impurity (Non-patent Literatures 1 and 2) and a relevance with a screw dislocation (Non-patent Literature 3) have been pointed. A mechanism of forming a nanovoid, however, has not completely been clarified.
For example, Non-patent Literatures 1 and 2 disclose that:
(a) when an Si impurity concentration or an O impurity concentration increases, the phenomenon of increasing a nanovoid density is observed; and
(b) on this occasion, obvious increase of dislocations is not recognized and hence an impurity relates to the formation of a nanovoid.
Meanwhile, Non-patent Literature 3 reports that a hollow screw dislocation exists in the center of a nanovoid, and a dislocation and a nanovoid relate to each other.
Non-patent Literatures 1 and 4 describe a relevance between a pit on a crystal surface (surface pit) and nanovoid forming. A nanovoid is thought to be formed from a surface pit, and hence a surface pit density is thought to have a strong correlation with a nanovoid density in a crystal. A surface pit, as described also in Non-patent Literature 5, causes various problems of making stable and long-time growth difficult while a bulk crystal grows, deteriorating crystallinity, and lowering a wafer processing yield. Further, when a power device is manufactured, an electric field concentrating part is generated from a surface pit as a starting point, and causes a withstand voltage to lower. Such surface pits therefore are required to be reduced to the greatest possible extent but an obvious countermeasure has not been clarified yet.
In an HVPE method or an MOCVD method, usually a member including a pBN coated graphite and a heater are used in a growth apparatus (Patent Literatures 1 and 2). A relevance between a B impurity derived from BN and a nanovoid, however, has not been known.
Patent Literature 3 describes that, when an impurity concentration is high, a crystal is likely to deform plastically and a crack is likely to be formed. As concrete substance names of impurities, however, only Mg, Fe, and the like are described and a B impurity is not referred to. Further, a relevance between an impurity and a defect such as a nanovoid has not been studied.
Further, in Patent Literature 4, there is a description of a defect called a “pipe hole” and a description that the density of “pipe holes” can be reduced to 300 pieces cm−2. The “pipe hole” described in Patent Literature 4, however, is a hollow defect existing right under an etch pit in the manner of penetrating a crystal, and is a defect essentially different from a “nanovoid” formed intermittently right under a surface pit.
That is, as shown in FIG. 1 in Patent Literature 4, from the facts that a “pipe hole” penetrates a crystal and a pit is formed by etching, a factor of forming a “pipe hole” is suspected not to be an impurity but to be a dislocation such as a hollow screw dislocation.
Moreover, Patent Literature 4 describes that “in the observation of a pipe hole, a large pit recognizable by an optical microscope is formed by etching a crystal with an acid or alkali solution”. On the other hand, when a crystal including a surface pit and a nanovoid is etched, the surface pit right above the nanovoid disappears by the etching and cannot be observed.